The present invention relates to a butterfly valve that is a kind of industrial valves used for the control of flow rate and pressure of liquid. In particular, the invention relates to a butterfly valve that can reduce the cavitation noise likely to occur in a small valve opening.
In a conventional butterfly valve, as shown in FIG. 1, a flow rate of the valve increases in a narrowed portion or a narrow area at an orifice side and a pressure around the valve lowers. The lowering of the pressure quickly increases the volume of each of fine air bubbles (bubble nuclei) contained in the liquid thereby generating cavitation bubbles. After liquid passes through the narrow area within the valve, the velocity of the flow decreases and recovers. Therefore, cavitation bubbles collapse in the downstream of the valve.
When the cavtation air bubbles grow from the air bubble nuclei around the valve, the peripheral pressure varies according to the contraction movement of the air bubbles, and this movement causes a tremendous noise. Further, the collapse of cavitation bubbles causes an impact pressure and the so-called cavitation phenomenon occurs that gives damages to the valve or piping arrangements by noise or vibrations.
Furthermore, the butterfly valve used for control aims originally at adjusting the flow rate or pressure by throttling the valve opening, and to achieve the object there is current no alternative but to use the butterfly valve while keeping patience on a cavitation accompanied by noise.
In light of the above present status there have been so far proposed butterfly valves for controlling a noise and cavitation to be produced from valve bodies.
FIGS. 2A and 2B show one example (see Japanese Patent Unexamined Publication No. 57-157866) of such a butterfly valve. The reference numeral 1 designates a valve disc which is disposed within a valve casing 2 and is pivotally supported by a valve rod 3 orthogonally crossing at the centre axis of the valve casing 2. The valve disc 1 is shown in the fully closed state in the drawing. The valve disc 1 is provided with a valve disc 1a in an angled form, being vertical to the valve rod 3. The peripheral surface of the valve disc 1a is cooperated with the inner surface 2a of the valve casing 2 to form a sealing surface which is shown with dotted line 1b in the drawing. The centre axis 1c of the sealing surface 1b passes through bores 4 provided on the valve disc 1 for receiving the valve rod 3. The centre axis 1c is inclined making an angle of about 15xc2x0 to 20xc2x0 relative to an axis 2b vertical to the inner surface 2a of the valve casing 2.
The valve disc 1a is intended to be in close contact with the inner surface or bore 2a of the valve casing 2 of the butterfly valve and is in such configuration that two semi-circular wall portions make an angled form as described above. Over one semi-circumference of the valve disc 1a are provided comb-teeth shape projections 5 at the flow-in side so that they are integrally projected in the flow-in direction (shown with thick arrow mark F1) at use. The other semi-circumference are provided with comb-teeth-shape projections 6 at the flow-out side which are integrally projected in the flow-out direction of the fluid i.e. in the reverse direction to the comb-teeth-shape projections 5 at the flow-in side. Both the comb-teeth-shape projections 5 and 6 are formed so as to be approximately in parallel with the inner surface 2a of the valve casing 2, and have tip ends formed so as to be positioned with the surface vertical to the inner surface 2a of the valve casing 2. Further, each of the comb-teeth-shaped projections 5 and 6 is formed to become gradually shorter towards bosses 7 each of which is positioned at the centre axis and into which the bores 4 are formed.
In the operation of the illustrated valve, if the valve disc 1 is rotated clockwise as shown with an arrow F2 from the fully closed state shown in FIG. 2A, the flow rate varies with the valve opening. At that time, the fluid passes through a nozzle side area and an orifice side area. The orifice side area is an opening portion delimited by the inner surface 2a of the valve casing 2 and the peripheral portion of the valve disc 1 positioned in the downstream side of the valve rod 3. The nozzle side area is an opening portion delimited likewise by the inner surface 2a of the valve casing 2 and the peripheral portion of the valve disc 1 positioned in the upstream of valve rod 3. The fluid flow passing through the nozzle side area and the orifice side area changes to fine jet streams by trapezoidal (in section) passages 8, as shown in FIG. 2B, formed between the plural respective comb-teeth-shaped projections 5 and 6. Consequently, the cavitation generated in the downstream side of the valve body 1 is dispersed to suppress the growth of the cavitation. In FIG. 2B the reference numeral 9 denotes inlets.
FIGS. 3A and 3B show another example of a conventional butterfly valve with a cavitation control function (see Japanese Patent No. 2536329). In a valve casing 11 a valve disc 12 is rotatably supported by valve rods 13 which are mounted on its two sides. When the valve disc 12 rotates from its closed state towards its opening, one semi-circumferential portion 12A of the valve body 12 moves towards the upstream side while the other semi-circumferential portion 12B moves towards the downstream side. In the surface near the outer periphery in the downstream side of the semi-circumferential portion 12A there are continuously projected approximately equally thicked ribs 14 over the whole semi-circle connecting the two supports 15 of a valve rod 13. Then, the respective ribs 14 are provided with a plurality of through bores 16 which inverse-radially converge being directed towards the surface centre axis 12C of the valve disc 12.
FIGS. 4A and 4B show still another example of a conventional eccentric butterfly valve (see Japanese Patent Publication No. 52-33330). The butterfly valve comprises a valve casing including a casing body 21 and a casing flange 22. A valve disc 23 is supported by a valve rod or biassed trunnion 24 attached to the casing body 21 in a position offset from the disc 23. The valve seat portion of the eccentric butterfly valve is provided with a resilient seating ring 25 in such a manner that the ring may be embedded in an inner wall recess 26 of the casing body 21.
The casing body 21 is provided with recesses 21a and 21b in the downstream side of the valve seat. As illustrated in FIG. 4B, the recesses 21a and 21b are formed to have a maximum depth in the intermediate position between bearing portions at both the sides of the biassed trunnion 24 and zero depth in the bearing portions at both the sides.
Then, it is arranged that at the rotation of the valve disc 23 the sectional area in the flow route takes place in almost the same proportion. herefore, the fluid torque applied to the disc 23 is reduced compared with a torque generated during the releasing motion of the valve in case the disc is vertically disposed relative to a cylindrical bore.
With the butterfly valves illustrated in FIGS. 2A, 2B, 3A and 3B, in the body casing or valve body side there are provided projections or grooves for defining fluid flow passages, or holes or additional elements which shut out the fluid flow line, thereby separating the flow of the fluid. Therefore, there is reduced the difference in velocity between a narrowed portion where a fluid flows at a high velocity and a low velocity flow portion where a fluid flows at a low velocity, and the cavitaton and thus the noise are controlled or suppressed.
However, such butterfly valves of the known structure have common problems as described below.
(i) The flow route is finely separated by a plurality of through bores which inverse-radially converge, the bores being provided in the ribs provided in the comb-shaped body in FIG. 2 or provided over the semi-circumference in FIG. 3. Therefore such butterfly valves are not usable for a fluid containing slurry or foreign particles, being feared for blocking with foreign particles.
(ii) Since, as described above, projections or additional elements are exposed to the flow, it is difficult to maintain their strength against the fluid force at high velocity or the collision by foreign particles.
(iii) The valve disc or the valve casing has a complicated shape, so that the manufacturing cost is high.
Further, the butterfly valve shown in FIG. 4 is not intended to restrain the cavitation or noise. That is, the recesses in the downstream side of the valve seat are not cylindrical being narrowed for width in the bearing portions so that the sectional area of the flow route becomes constant at the rotation of the valve disc thereby to aim at the reduction of torque.
It is, therefore, an object of the present invention to overcome the various problems in said conventional techniques and to provide a butterfly valve with low noise and with simple construction, which is capable of controlling any cavitation occurring in the narrowed flow region of the valve whereby a noise is reduced.
Another object of the present invention is to provide a butterfly valve simply arranged so that a forced partial stress is not applied to a high flow velocity energy or the collision by foreign particles so as to be used for fluids intended to be transported at a high flow rate and for fluids even containing slurry or foreign particles.
To achieve the above objects according to the present invention there is provided a butterfly valve with low noise comprising:
a valve casing;
a valve disc which is rotatably supported by a valve rod in the valve casing and which opens and closes a fluid flow line in cooperation with a valve seat provided in an inner peripheral wall of the valve casing; and
an enlarged section which is provided on the valve casing in a downstream side immediately behind the valve seat provided on the inner peripheral wall of the valve casing and has an enlarged sectional shape symmetrical with respect to a centre axis for controlling a noise and cavitation to be generated on the valve disc.
With the butterfly valve according to the present invention, the provision of the enlarged section makes it possible to lower a flow rate of a fluid flowing in the narrowed flow region which is defined in the downstream side immediately behind the valve seat at the valve opening so as to prevent a pressure of the fluid from being reduced. Further, the enlarged section is axially extended to increase the area of high pressure in the downstream of the valve disc. It is, therefore, possible to reduce a chance of growing an air bubble nuclei based on the shearing force by the flow rate difference, and to shorten the time of growing air bubble, followed by control of growth of the air bubbles.
In order to control the occurrence of the noise and cavitation, as a result of practical measurement it has been found that a beginning portion of the enlarged section may be positioned at a distance which is smaller than xc2xd of an inner diameter of a piping to which the butterfly valve is mounted from a position immediately behind the valve body in a fluid flow direction. The beginning portion of the enlarged section may be most preferably determined at the position immediately behind the valve body. It is also found that an inner diameter of the enlarged section may be more than 1.3 times the inner diameter of the piping while the axial length of the enlarged section may be more than 0.5 times the inner diameter of the piping.
Moreover, in order to control or suppress the noise and cavitation to be generated in the valve disc the outer diameter of the valve disc and the inner diameter of the valve seat of the valve casing may be made lower than 0.77 times the inner diameter of the piping. In this case, the inner diameter of the enlarged section may be made to be the size same as the inner diameter of the piping.